1. Field of the Disclosure
This disclosure relates to creatinine biosensors and the uses thereof. More specifically, this disclosure describes potentiometric creatinine sensors which utilizes one or both of a type of enzyme capable of directly producing ammonium ions (NH4+) as a consequence of coming into contact with a liquid sample and an internal fill solution with a low free ammonia ion concentration.
2. Brief Description of the Related Art
The presence of creatinine in a patient's blood may, for example, function as an indicator of the patient's renal health as creatinine is a byproduct of muscle metabolism. As such, creatinine levels in a patient's blood sample may be used to assess kidney health as well as how the effectiveness of medication(s). Standard multi-enzyme amperometric creatinine biosensors produce and then measure an electrical current which flows between the biosensor and the blood sample the biosensor is immersed in. Once measured, the resulting current can be used to determine the concentration of creatinine in the blood sample. The current is measured using an active electrode (also referred to as a working electrode) and an inactive electrode (which may also be referred to as a compensation electrode). The electrical current is a byproduct of an enzymatic cascade reaction which begins when the active electrode is inserted into the blood sample.
In a typical example of a standard amperometric creatinine biosensor, three enzymes are immobilized onto the surface of the active electrode: Creatinine Amidohydrolase, Creatine Amidohydrolase, and Sarcosine Oxidase. When the active electrode is inserted into the blood sample, the following cascade reaction occurs: (1) the enzyme Creatinine Amidohydrolase reacts with Creatinine and H2O in the sample to produce Creatine; (2) the enzyme Creatine Amidohydrolase reacts with the Creatine to produce Sacrosine and Urea; and (3) the Sarcosine Oxidase enzyme reacts with the Sarcosine as well as O2 and H2O in the sample to produce Glycine, Formaldehyde and H2O2—which then results in the creation of an electrical current.
This cascade reaction associated with the active electrode may be expressed as follows:

In order to compensate for latent amount of creatine present in the blood sample prior to cascade reaction, the two enzymes are immobilized onto the surface of the inactive electrode: Creatine Amidohydrolase and Sarcosine Oxidase. When the inactive electrode is inserted into the blood sample, the following cascade reaction occurs: (1) the enzyme Creatine Amidohydrolase reacts with the Creatine already present in the blood prior to the insertion of the active electrode to produce Sacrosine and Urea; and (2) the Sarcosine Oxidase enzyme reacts with the Sarcosine as well as O2 and H2O in the sample to produce Glycine, Formaldehyde and H2O2— which then results in the creation of an electrical current. By comparing the resulting electrical current recorded by the active and the inactive electrodes, the creatinine concentration in the sample can be determined.
The cascade reaction associated with the inactive electrode may be expresses as follows:

Multi-enzyme amperometric creatinine biosensors have several drawbacks. First, immobilizing three different types of enzymes onto the active electrode and two different types of enzymes onto the inactive electrode complicates manufacturing. Second, during use, the accuracy of the device can be compromised by a failure of just a single enzyme. Such a failure may result manufacturing defects, material defects, contamination, interfering species, etc. . . . . Third, once produced, the shelf life of these biosensors can be short (for example a matter of weeks).